Protective circuit for superconducting magnet



July 29, 1969 J. E. KUNZLER PROTECTIVE CIRCUIT FOR SUPERCONDUCTINGMAGNET Filed April 12, 1967 /N I/EN TOR J. E. KUNZL ER A TTQRNEK UnitedStates Patent 3,458,763 PROTECTIVE CIRCUIT FOR SUPERCONDUCTING MAGNETJohn E. Kunzler, Pleasant Grove, N..I., assignor to Bell TelephoneLaboratories, Incorporated, Murray Hill, N.J., a corporation of New YorkFiled Apr. 12, 1967, Ser. No. 630,257 Int. Cl. H0211 7/06, 7/08, 7/10US. Cl. 31713 9 Claims ABSTRACT OF THE DISCLOSURE A secondary winding isclosely coupled to the windings of a superconducting magnet in acryostat containing liquid helium. An appreciable thermal mass ispositioned above the level of the liquid helium and is connectedelectrically to the secondary. The thermal mass serves as a heatexchanger that is cooled by gaseous helium whenever excess andpotentially destructive energy produced by a sudden collapse of themagnetic field is transferred from the magnet to the secondary andthence to the thermal mass.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to superconducting electromagnets and more particularly toprotective arrangements for dissipating the excess and poentiallydestructive energy that is released by the relatively sudden collapse ofthe superconducting magnets magnetic field.

Description of the prior art It is known that high strength magneticfields may be produced by means of superconducting electromagnets thatare constructed by coiling one or more turns of a superconducting wireor other superconducting element about a common core. The magnetstructure is typically immersed in a low temperature environment such asliquid helium, for example, in a suitable insulating container such as aDewar flask or cryostat. The low temperature environment is essential tomaintain the magnet coil in its superconducting state. These magnetshave the advantage of compactness and practically negligible powerrequirements. Persistent currents can be established in the coils owingto their superconducting properties and hence the power supply need beconnected to the coil only for initial energization or, at most, onlyintermittently. A complete stable magnetic field can thus be maintainedso long as the coils are kept at or below their critical temperature andcritical field.

When superconducting magnets are operated at high magnetic fields, theyare particularly susceptible to an accidental collapsing of the field. Afield collapse of this nature may occur within a fraction of a secondand can result in a destructive shock Wave. Additionally, if the magnetis sufi'iciently large and the field sufiiciently high, the energyinduced in the coils by the collapsing field may be converted into heatenergy great enough in magnitude to melt, vaporize or even explode themagnet structure.

In order to provide a measure of protection against the inherent dangersof a suddenly collapsed magnetic field, a number of prior artsuperconducting magnet systems employ some means of dissipating theexcess energy created thereby. In the case of relatively small magnets,for example, it has been possible to connect an energy dissipating ordumping circuit external to the cryostat- Energy from the magnet coilmay be transferred to a closely coupled secondary coil within thecryostat and from the secondary coil to the external dumping circuit byway of connecting leads.

In the case of particularly large magnets, a protective arrangement ofthe type indicated requires the use of relatively heavy conductorsbetween the secondary coil and the dumping circuit in order to avoidexcessive inductive voltages. Conductors of such magnitude, however,also conduct heat into the cryostat during normal operation and as aresult, an unacceptable amount of helium can be lost through a more orless constant boiling-oft process.

SUMMARY OF THE INVENTION The object of the invention is to avoid thepotentially damaging effects of suddenly collapsing magnetic fields insuperconducting magnets while still maintaining the attendant loss ofthe cooling medium at an acceptable level. This object and relatedobjects are achieved in accordance with the principles of the inventionby providing a superconducting magnet arrangement with a closely coupledsecondary winding which has a time constant (L/R) of sufiicient lengthto allow a relatively gentle decay of the field. Energy coupled into thesecondary winding is in turn conducted to a resistive member within thecryostat. The resistive member is characterized by a large thermal massthat is positioned above the level of the liquid cooling medium toensure minimum thermal contact therewith.

In accordance with an important aspect of the invention the thermal massoperates effectively as a heat exchanger in absorbing the heat energythat results from a suddenly collapsing field. In the event of a fieldcollapse, a portion of the attendant heat energy that appears in thesecondary winding and in the magnet structure vaporizes a small amountof the cooling medium. The resulting low temperature gas is directedagainst the thermal mass resistive element as a coolant before it ispermitted to escape through the cryostat vent. In this way, thepotentially destructive release of heat energy is effectively andgradually dissipated and the magnet structure is fully protected fromdamage.

BRIEF DESCRIPTION OF THE DRAWING The single figure of the drawing is asketch, partially in cross section, of a superconducting magnet assemblyin accordance with the invention.

DETAILED DESCRIPTION In the figure a superconducting magnet structure101 is formed from a superconducting coil 102 around a core member 103.The coil 102 may be formed from any suitable superconducting materialsuch as one of the many niobiumzirconium alloys. A number of specifictypes of superconductive magnet coil structures are known in the art,one illustrative structure being shown in Patent 3,129,359, issued to J.E. Kunzler, on Apr. 14, 1964.

The magnet structure 101 is immersed in a cooling medium 105, such asliquid helium, for example, to ensure the maintenance of a temperatureless than the superconducting temperature level that is peculiar to thepar ticular material of the coil 102. The magnet structure 101 and theliquid helium bath 105 are contained within a cryostat 104 to ensuremaximum isolation from ambient temperatures.

In accordance with the invention a secondary winding assembly 106 isemployed to dissipate energy that is inductively transferred theretoupon any sudden collapse of the field of the magnet structure 101. Thesecondary assembly 106 includes a first coil 107 that is closely coupledto the coil 102, to ensure efficient energy transfer from a collapsingfield.

The time constant characterized by the L/R ratio of the coil 107 shouldbe kept as long as practicable to extend the time duration of energytransfer. A further requirement for the coil 107 is that its resistanceshould be as low as practicable to minimize the translation of currentinto heat energy at that point. Accordingly, the coil 107 should beformed from a high conductivity material such as aluminum or copper,aluminum having the advantage of low magnetoresistance.

Leads 111 are employed to connect the secondary coil 107 to a heatexchanger coil 108 and to a large thermal mass resistive heat sink 109which is in form similar to an inverted funnel. Alternatively the coil108 and the heat sink 109 may be replaced by a common heat sink mass.The combined material of the coil 108 and the heat sink 109 should besufficient to limit heating to less than several hundred degrees C. whenall of the energy from a collapsing field is dumped into it. In order tobe reasonably effective, however, the temperature rise of the heat sink109 should be at least a substantial fraction of 100 K.

The combination of the thermal sink 109 and the coil 108 operates, ineffect, as a heat exchanger which is cooled by escaping helium gas 110which is forced to pass through the heat exchanger. The spacing betweenthe rim 113 of the mouth portion of the funnel-like heat exchanger 109and the wall of the cryostat 104 is sulficiently limited to ensure thatmost of the gas reaches the neck portion or vent. The resistance of theheat exchanger combination (coil 108 and heat sink 109) is made verylarge compared to the secondary winding 107 in order to ensure that mostof the translation from electrical energy to heat energy takes placeoutside of the liquid helium bath 105. In this connection, designconsideration must be given to the increased resistance of the coil 107that results from the heat generated therein during the collapse of themagnetic field. In accordance with the invention, suitable alloys may beselected to term coil 108 and the heat sink 109 in order to achieve theappropriate combination of resistance and thermal mass.

SUMMARY OF OPERATION OF THE EMBODIMENT Whenever the coil 102 is drivennormal (i.e., becomes suddenly nonmagnetic), much of the energy in theresulting collapsing field is transferred to the secondary winding 107,the L/R time constant of which should be large compared to that of thewindings 102 in their normal or nonsuperconducting state in order toensure efficient coupling. The rate of collapse of the field iscontrolled by the time constant of the secondary windings 107. Sincemost of the resistance of the secondary winding assembly 106 is in thewinding 108 and in the heat sink 109, most of the heat will be generatedthere. Owing to the fact that the coupling between the windings 102 and107 is not ideal, however, some heat is developed within the magnetstructure 101. This heat evaporates a certain amount of liquid helium.The gaseous helium passes through the heat exchanger consisting of thecoil 108 and the heat sink 109, thus cooling these elements. Evenassuming that the coupling between the coils 102 and 107 were toapproach the ideal, and the resistance of the coil 107 were negligibleas compared to the heat exchanger elements 108 and 109, heat would stillflow from the thermal sink 109 down the leads 111 into the helium bath105, thus providing the gas 110 for removing heat from the heatexchanger 109.

It is to be understood that the embodiment described herein is merelyillustrative of the principles of the invention. Various modificationsthereto may be effected by persons skilled in the art without departingfrom the spirit and scope of the invention.

What is claimed is:

1. Superconducting magnet apparatus comprising, in combination, a firstcoil of superconductive material, a secondary coil closely coupled tosaid first coil but insulatedly separated therefrom, a liquid coolingmedium for maintaining said first coil below its superconductingtemperature, said first coil and said secondary coil being immersed insaid liquid, an insulated container for maintaining said liquid at asubstantially fixed temperature, a highly resistive heat sink positionedin said container above the level of said liquid, and conductive meansconnecting said secondary coil to said heat sink, whereby energy coupledinto said secondary coil by any sudden collapseof the field of saidsuperconducting magnet is translated into heat energy in said heat sink,said heat sink being cooled by gas resulting from the partialvaporization of said liquid.

2. Apparatus in accordance with claim 1 wherein the electricalresistance of said heat sink is substantially higher than the electricalresistance of said secondary coil.

3. Apparatus in accordance with claim 1 wherein said heat sink comprisesan inverted funnel-shaped member with the neck portion thereof forming avent for said container.

4. Apparatus in accordance with claim 3 wherein said heat sink furtherincludes a relatively high resistance coil positioned in said neckportion of said funnel and electrically connected thereto.

5. Superconducting magnet apparatus comprising, in combination, a firstcoil of superconducting material, a secondary coil closely coupled tosaid first coil but insulatedly separated therefrom, a cryostat and aliquid coolant contained therein, said first coil and said secondarycoil being immersed in said coolant, means within said cryostat abovethe level of said liquid for translating electrical energy into heatenergy, and electrically conductive means connecting said secondary coilto said translating means, said translating means being positioned topermit the cooling thereof by vaporized portions of said liquid.

6. Apparatus in accordance with claim 5 wherein said liquid is helium.

7. Apparatus in accordance with claim 5 wherein said translating meanscomprises electrically conductive material having a relatively highthermal mass.

8. Apparatus in accordance with claim 7 wherein said translating meanscomprises an inverted funnel member with the neck portion thereofforming a vent for escaping helium gas.

9. Apparatus in accordance with claim 8 including a relatively highresistance coil positioned in the neck portion of said funnel member,said last-named coil being electrically connected to said funnel member.

References Cited UNITED STATES PATENTS 3,177,408 4/1965 Mills et al.317-123 3,183,413 5/1965 Riemersma et al. 317-123 3,270,247 8/1966Rosner 317--13 3,412,320 11/1968 Marshall 335216 X LEE T. HIX, PrimaryExaminer US. Cl. X.R. 335-216

